Due to the general trend from analogue techniques to digital techniques and from hardware to software, press key switches, combined individually or in keyboards, are increasingly used in electrical engineering equipment, communications engineering equipment and, last but not least, in so-called terminals or data input devices. There are a large number of different designs and functional principles for keyboards of this type, which represent the interface between man and machine. The nature of the travel is very important for reliability and also for man--keyboard contact. In addition to the so-called flat or membrane keyboards with a travel of up to about 1.5 mm, there are also, depending on the field of application, keyboards with a travel of between 1.5 and 6.35 mm. In addition, keyboards with touch switches are of course also used. All keyboards with travel have in common, however, that the operating person should, by the sense of touch, obtain a kind of feedback, confirming that the particular key has been actuated.
German Offenlegungsschrift No. 2,418,583 a keyboard in which snap elements, which consist of a rubber-elastic material having the shape of a table mountain, are arranged above a printed circuit board provided with tracks. The section located above the contact points of the printed circuit board is joined to the remaining part by a wall which can be folded or tilted. The design of the wall is such that its thickness gradually decreases from an upper section towards a middle section and gradually increases from the middle section to a lower section having a curved surface which is inclined inwards into the interior of the snap element and extends from the middle section to the upper section. A restoring force is thus obtained by deformation of the wall, the operating person feeling a kind of snapping effect.
So-called switch mats of non-conductive silicone rubber are also known for keyboards: these have a rubber of cupola-shaped raised portions. One contact element of conductive silicone rubber, interacting with contact surfaces of a printed circuit board, is arranged in the hollow interior of each such raised portion. A switch mat of this type is known, for example, from German Offenlegungsschrift No. 3,029,515. Stationary contacts are arranged on a dimensionally stable, electrically insulating base plate. The actual switch mat with a sheet-like composite body and the cupola-shaped raised portions are located above this base plate. The switch mat itself is produced by compression-moulding, vacuum-forming or injection-moulding from a silicone rubber mixture which consists of a silicone rubber and a crosslinking agent. The contact element is produced from a commercially available electrically conductive silicone rubber composition and is joined by vulcanisation to the inner wall of a raised portion. Alternatively, the contact element can be moulded from an electrically conductive plastic, or it can be produced by printing, transferring or coating an electrically conductive composition onto the carrier body.
A further switch mat and process for its manufacture are described in German Offenlegungsschrift No. 2,902,892. Initially, the contact elements are applied to a carrier. In doing this, a uniform layer of an uncrosslinked, electrically conductive elastomer mixture is applied to the carrier. Under the action of pressure and heat, this layer is deformed in a mould to give the desired structure of the contact elements, complete or partial crosslinking taking place. The mould has narrow upright edges which correspond to the contact element. The contour rims on the mould and the carrier mate such that when the mould is closed the elastomer material is forced out between the surface of the carrier and the end faces of the contour ridges or upright edges of the mould. After the mould has been opened, only the contact elements remain on the carrier. On top of this, an electrically insulating uncrosslinked elastomer mixture which is intended to form the composite body is then applied. Subsequently, cross-linking is carried out under pressure and with the action of heat. If the same elastomer is used as the base material for the contact elements and for the composite body, they can both be joined to one another without an adhesion-promoter to give a unitary body. If different base materials are employed, the use of an adhesion-promoter or of an adhesive is necessary. In a final step, the carrier is removed from the switch mat.
Another process for the manufacture of a switch mat is known from German Offenlegungsschrift No. 2,822,033. A ribbon of electrically conductive silicone rubber is drawn through a suitable gap in an injection mould, and the contact elements are punched out of the ribbon by means of a punch and are each moved to a mould cavity. Subsequently, non-conductive silicone rubber is injected via an injection runner into the heated mould. After full vulcanisation, a firm joint between the contact element and the composite body is produced. After the mould halves have been opened, the finished switch mat is ejected by means of compressed air.
All of the above described processes involve undue complexity and waste.